"The African National Congress Gauteng is shocked and distraught by the killing of Comrade Jackson Mafika by unknown gunmen at his... home on Wednesday," it said in a statement.
"The ANC has lost a great leader who has been committed to serving the people."
Police spokesperson Elsie Tshonte confirmed that a man was shot on Wednesday, but could not provide further details.
The ANC said Mafika had been preparing for the burial of his brother, who was stabbed last week.
"Comrade Jackson was an astute leader, who served with great diligence and commitment in the Westonaria municipality as the ANC chief whip and member of the mayoral committee on finance," it said.
"This is a tragedy of great proportion, that our leader should be killed in such mysterious circumstances. We are in a democracy, it should not be easy to kill anyone, hence we call upon the police to swiftly investigate the killing."
The party sent its condolences to Mafika's family.